The present disclosure relates to a changeover method using a changeover logic of at least two systems of a high voltage direct current (HVDC) transmission system.
There are two power system interconnection methods, a typical one for interconnecting AC power systems as they are, and the other for interconnecting systems after converting AC power into DC power through a power converter. In recent, an interest in the method of interconnecting power systems after converting the AC power into the DC power is more increasing than the method of interconnecting AC power systems as they are. A high voltage direct current (HVDC) transmission system using a power converter is also installed between Jeju-island and Haenam-gun in Korea to interconnect power systems between Jeju-island and Haenam-gun.
An HVDC transmission method is one of electricity transmission methods and indicates a supply method that converts high voltage AC power generated at the power station into DC power, transmits the DC power and then re-converts into AC power in a desired power receiving region.
The DC transmission method has many advantages.
Firstly, since the size of a DC voltage is just about 70% of the maximum value of an AC voltage, the HVDC transmission system may easily perform device insulation and since there is a low voltage, it is possible to reduce the number of insulators installed in each device and the height of an iron tower.
Since the DC transmission method has less transmission loss than an AC transmission method when the same power is transmitted, the best advantage of the HDVC transmission system is that power transmission efficiency may increase. The HDVC transmission system may transmit two or more times the current of the AC system in the DC system.
Since the HVDC transmission system may reduce wire consumption and the area of a transmission line, it is effective and also possible to enhance the stability of systems by connecting two systems having different voltages or frequencies.
The HVDC transmission system has no constraint on a power transmission distance and the DC transmission method needs inexpensive construction costs in land power transmission exceeding 450 Km or in submarine power transmission exceeding 40 Km as well.
Thus, the HVDC transmission system is used for a power system interconnection method of new renewable energy, especially for power transmission of a large offshore wind farm.
Since in other countries, such as China and India, the distance between a power station and an electricity user is 1000 Km or longer, the prevalence of the HVDC transmission system is rapidly increasing.
Components in the HVDC transmission system including a controller are provided in pairs for the stability of the system, in which case one of a pair of devices is in an active state and the other of the pair of devices is in an inactive state.
An operation that the device being in the active state between a pair of devices changes to the inactive state and simultaneously, the device being in the inactive state changes to the active state is called changeover.
In a typical changeover technique, even if a small glitch or noise occurs on a line to which a signal is input, a changeover logic (COL) obtaining a fault signal changes over the controller.
There is a case when a system actually having a fault transmits the fault signal to the COL, but a normal signal transmitted from a system is deformed to the fault signal and transmitted even when a wire or line transmitting a signal has a physical trouble and thus a glitch or noise occurs.
In this case, the COL obtaining the fault signal determines that the system transmitting the fault signal has a fault.
Thus, since a typical system is vulnerable to the fault, and the COL immediately receives a command provided by the controller and performs a changeover operation, the entire HVDC transmission system excessively depends on the controller capable of malfunctioning, thus unstable and experiences runout.